Stemmed instruments of this type comprise a distal tool and a proximal drive unit. A stem extends between the operational tool and the drive unit and receives a longitudinally displaceable force-transmitting element. The distal end of the force-transmitting element acts on the operational tool and the proximal end engages the drive unit. When a tensile load is applied by actuating the drive unit, the force-transmitting element is shifted inside the stem and thereby actuates the operational tool.
Furthermore, an overload protection device is present in the force-transmission path to limit the transmissible tensile force. This overload protection device is designed to protect the apparatus from damage in the event of excessive traction applied through the drive unit, in particular from destroying the operational tool. Excessive tensile stress of an operational tool, for instance in the form of tongs of which the bearing shaft would break, absolutely must be precluded. If such a rupture were to occur during surgery, there would be danger that the tong elements, no longer securely held, would drop and might seriously injure the patient.
In this respect, it is known from German patent 1,964,896 to use a coupling with coupling elements as the overload protection device, where the coupling elements are kept magnetically engaged. Retention by the magnet used is selected in such manner that if the set limit force is exceeded, the coupling elements disengage from each other, interrupting the force transmission between the drive unit and the operational tool in an impulsive manner. This known apparatus entails complex manufacture.